Political crusaders have long sought to lift up the downtrodden, comfort the afflicted, and empower the defenseless. Scott Crider, the founder of Dogs Against Romney, says he fits squarely into this grand tradition.

His mission: Make sure every American voter knows that once, 29 years ago, Mitt Romney drove for 12 hours to Canada with the family’s Irish setter, Seamus, in a crate on the roof.

“I’m just a guy who is trying to give a voice to the voiceless,’’ said Crider, a 47-year-old from Gulf Shores, Ala., who says his unstinting dedication to the Seamus story has become a “24/7’’ pursuit that has crowded out his day job as a marketing consultant.

“For me, this dog thing is a deal-breaker,’’ he said. “I’m doing it because I genuinely feel like this speaks to his character.’’

For anyone who has not read a Gail Collins column in the New York Times, seen the cover of the March 12 New Yorker, or happened across a late-night comedy bit, the tale of Seamus has become one of the enduring narratives of the 2012 presidential campaign.

As originally recounted in a 2007 Globe series about Romney’s life, Romney put Seamus in a crate with a windscreen atop the family’s station wagon for a summertime ride from Boston to a lake house in Ontario.

Sometime during that sojourn, however, Seamus soiled the crate and the car’s rear window. Romney pulled into a gas station, hosed down Seamus, and returned him to his rooftop carrier for the duration of the trip.

The Globe cited the anecdote as an example of Romney’s “emotion-free crisis management.’’

Crider did not see it that way.

“It struck me as really an odd and very cold thing to do to a pet,’’ he said.

A day after he read the account in 2007, a movement was born.

Crider launched a blog about Seamus, written from the perspective of Rusty, a slightly startled-looking brown mutt, who asks readers to “help me get my message out’’ that “Mitt Romney is mean to dogs.’’ Crider says his goal was just to entertain his friends, but the blog was picked up by news sites.

Within 10 days, he said, the site had attracted more than 1 million visitors.

“It has just been a steady upward growth since then,’’ he said.

The Dogs Against Romney Facebook page now has more than 38,000 members. The blog, he said, draws between 4,000 and 6,000 visitors every day.

Its followers have taken to the streets. Last month they protested outside the Westminster dog show in New York, carrying signs that read,“Dogs aren’t luggage’’ and “I ride inside!’’

Last week they demonstrated outside the Humane Society’s “Bark in The Park’’ expo in Tampa. On Monday they plan to protest outside a Romney fund-raiser in Peoria, Ill. Crider has also started a petition to compel Congress to “Ban Animals on Roofs of Cars (B.A.R.C.).’’

“It obviously resonates with a lot of animal lovers and pet owners,’’ said Tim Heberlein, a 29-year-old consumer rights activist who attended the Dogs Against Romney rally in Tampa. “I have two cats,and I definitely wouldn’t strap them to the top of my car.’’

A minor political crisis beset the group when it was revealed that Americans United For Change, a liberal group, helped publicize the protest at the Westminster dog show, undercutting Crider’s claim that he is driven by purely nonpartisan canine-centric concerns.

But Crider, who owns a 13-year-old chocolate lab named Cocoa, insists he was just happy for the publicity. A self-described independent voter with moderate views, he pointed out that aides to Rick Santorum and Newt Gingrich have also latched onto the Seamus story to cast Romney as inhumane.

Romney has denied any mistreatment of Seamus.

“This is a completely airtight kennel, mounted on the top of our car,’’ he told Fox News last year. “He climbed up there regularly, enjoyed himself.’’ With five children in the car, he added, “my guess is he liked it a lot better in his kennel than he would have liked it inside.’’

Crider is not persuaded. Sounding every bit the activist committed to his cause, he said he is determined to keep dogging Romney with the story of Seamus.